The present invention relates to the generation of charged particles in air, and more particularly to the generation of charged particle images for electrographic imaging.
Charged particles for use in electrographic imaging can be generated in a wide variety of ways. Common techniques include the use of air-gap breakdown, corona discharges and spark discharges. Other techniques employ triboelectricity, radiation, and microwave breakdown. When utilized for the formation of latent electrostatic images, all of the above techniques suffer certain limitations in charged particle output currents and charge image integrity.
A further approach, which offers significant advantages in this regard, is described in Fotland, U.S. Pat. No. 4,155,093 (May 19, 1979) and the improvement disclosed in Carrish, U.S. Pat. No. 4,160,257 (Jul. 3, 1979). These patents disclose method and apparatus for generating charged particles in air involving what the inventors' term "silent electric discharge". The prior art general view of FIG. 1 shows a charge image generator 8 capable of forming an electrostatic latent image on electrostatic latent image receptor 25. Charge image generator 8 is supplied with a high voltage alternating potential from generator 10. This potential is applied between two electrodes, a generator electrode 12 and a control electrode 14. Electrode 14 contains a plurality of circular or slotted apertures opposing generator electrode 12. Solid dielectric member 16 is sandwiched between these electrodes. Generator electrode 12 is shown encapsulated by dielectric member 18. As disclosed in U.S. Pat. No. 4,155,093, the alternating potential causes the formation of a pool or plasma of positive and negative charged particles in the air region adjacent dielectric 16 and defined by the apertures in discharge electrodes 14. These charged particles may be extracted to form a latent electrostatic charge image.
The alternating potential supplied by generator 10 creates a fringing field between electrode 12 and electrode 14. When the electrical stress exceeds the dielectric strength of air, a discharge occurs in the fringing field air gap. Charge built up on the surface of dielectric 16 reduces the electric field in the air gap thus quenching the discharge. Such silent electric discharges produce a faint blue glow. In order that no discharge occur in the region between adjacent control electrodes in space 15, this region must be filled with a solid dielectric.
U.S. Pat. No. 4,160,257 teaches the use of isolation or screen electrode, 20, separated from control electrode 14 by spacer layer 22. Electrode 20 serves to screen the extraction electric fields in the region bounded by electrodes 14 and 20 from the external fields associated with the latent charge image formed on the surface of dielectric receptor 26. In addition, aperture 24 in electrode 20 provides an electrostatic lensing action. Passage of charged particles through isolation aperture 24 to the surface of image receptor dielectric 26 is controlled by electrical potentials applied control electrodes 14. The electrical potential of isolation electrode 20 is kept constant with time. The receptor dielectric is contiguous with conducting substrate 28. The edge of a second control electrode 17 is also shown in FIG. 1. The space electrically isolating control electrodes must be filled with a solid dielectric 15 to prevent air gap breakdown in this region.
The use of negative charges (electrons and negative ions) is preferred since higher negative output currents are obtained than when potentials are reversed to extract positive charges. Biasing power supply 34 provides a constant high-voltage accelerating field between dielectric receptor substrate 28 and isolation electrode 20. Negative charges are extracted from the discharge when print selector switch 36 is in position Y. In this case, a charge extraction field, provided by power supply 30, is present between electrodes 14 and 20. When switch 36 is in position X, a retarding field is applied by supply 32 and the retarding field prevents charge from escaping aperture 24.
The requirement that a high frequency voltage and an extraction voltage be simultaneously present to generate charge output provides the means for coincident selection thus enabling the multiplexing of charge output. The prior art view of FIG. 3 illustrates how the charged particle generator 57 may be multiplexed. An array of control electrodes 58-1 through 58-6 contains apertures 62 at crossover regions opposing generator electrodes 60-1 through 60-4. Dielectric layer 64 isolates generator and control electrodes. Isolation electrode 66 is contiguous with dielectric layer 64. Generator electrodes are sequentially excited by a high frequency high voltage burst of several cycles. Any location in the matrix may be printed by timing a data, or control, pulse to the selected control electrode simultaneous with excitation of the appropriate generator line.
Two methods of fabricating charge image generators are described in the patent literature. One method involves first forming a laminate consisting of discharge dielectric 16 sandwiched between metal foils which are subsequently chemically etched to form generator electrodes 12 and control electrodes 14. After etching, the generator electrode side of the laminate is bonded to dielectric 18 which, in turn, is bonded to a metal heat sink not shown in FIG. 1. The photo-etched laminate is then laminated, on the control electrode side, with a photo-etchable dry film soldermask or dry film photoresist. Next, openings are formed in spacer layer 22 to expose the apertures previously etched into the control electrodes. Finally, a previously etched isolation, or screen, electrode 20 is bonded to the spacer layer. Briere U.S. Pat. Nos. 4,381,327 and 4,628,227 and Fotland et al, U.S. Pat. No. 4,408,214, incorporated herein by reference, describes this method in detail.
A second fabrication method involves building up the layers starting with generator electrode 12 that is formed on insulating support 18. Layers are subsequently fabricated sequentially on this generator electrode structure. This technique is described in detail in the following U.S. Pat. Nos.: McCallum et al. 4,679,060; 4,745,421; 4,958,172; 5,030,975 and Kubelik 5,315,324 which are also incorporated herein by reference.
Both fabrication approaches employ spacer layers 22 between about 50 microns and about 150 microns in thickness. Since bathtub shaped apertures must be formed in the spacer layer, this layer is formed of either a dry film photomask or a dry film photoimagable solder mask material. Two layers are required for thicker spacing. Alternately, this spacer layer may be formed using screen printing of the appropriate thickness curable resin.
The space between adjacent control electrodes must be filled with solid dielectric 15 in order to prevent air-gap breakdown in the fringing fields adjacent the edges of the control electrodes. Air gap breakdown in this region increases the power required to drive the charge image generator and eventually results in arcing and catastrophic failure as the insulation is eroded in the highly oxidizing environment created by the discharge. U.S. Pat. Nos. 4,679,060 and 4,745,421 show a method of reducing the magnitude of the control electrode edge sealing problem by including the extra step of coating the control electrodes and spaces between these electrodes with a 25 micron layer of liquid solder mask. The cured solder mask effectively seals space 15. A thicker solder mask film is then laminated to the cured solder mask and the finishing steps carried out.
When a separate and distinct sealing operation is not employed, the dry film solder mask must be laminated to the control electrode and surrounding dielectric using a vacuum laminator arranged to provide sufficient heat and pressure so that the semi-molten dry film solder mask will flow into the spaces between the control electrodes thus effectively sealing this region.
A second manufacturing problem encountered in the present fabrication schemes involves alignment of control electrode apertures with corresponding screen apertures. Alignment between the control electrode apertures and corresponding generator electrodes is relatively easy since alignment is only required in one direction because the generator electrodes are in the form of stripes. In addition, the stripe width is typically chosen to be somewhat greater than the control electrode aperture diameter. The screen and control electrode apertures, however, must be accurately aligned in two directions over the entire width of the charge image generator in order to provide uniform charge output.
Additional problems relate to yield reductions associated with the flimsy nature of various layers. In the first above described fabrication method, a rather delicate thin mica strip is laminated with two metallic foils and these foils are then photo-etched to form the control and generator electrode shapes. Exposure to liquid photo-etching processing sprays very frequently leads to mica cracking. These cracks, in turn, lead to early life catastrophic charge image generator failure. In the second fabrication method described above, the control electrodes are etched as free-standing foil supported at the edges with pressure sensitive tape. The use of tape to minimize distortion of the freestanding foils is the subject of U.S. Pat. No. 4,745,421.
Accordingly, it is a principal object of the invention to simplify the manufacturing process of charge image generators. A further object is to provide a manufacturing method having improved manufacturing yields. Related objectives involve improve operating characteristics by reducing catastrophic failures and improving charge output uniformity. A still further objective provides for charge image generator cost reduction, Also, the invention provides for improved heat transfer from active discharge areas.